This invention relates to methods and apparatus for combusting carbonaceous fuel, and more particularly for combusting such fuels in the presence of a catalyst to produce a hot gaseous effluent for use as a source of heat, as in a furnace or heat-treatment system, or as a source of power such as a motive fluid in a turbine system.
In U.S. Pat. No. 3,928,961, granted Dec. 30, 1975, in the name of William C. Pfefferle and assigned to the same assignee as that of the present invention, there is disclosed a process designated catalytically-supported, thermal combustion. According to this method carbonaceous fuels can be combusted very efficiently in the presence of a solid oxidation catalyst at thermal reaction rates but at temperatures below nitrogen oxide-forming temperatures. However, such combustion occurs efficiently only within a range of temperatures above an approximate minimum temperature, of the order of 1700.degree.-2000.degree. F, below which very substantial proportions of incompletely combusted fuel, usually in the form of hydrocarbons or carbn monoxide or both, may leave the catalyst to constitute a pollution nuisance. In fact, at the start-up of combustion the catalyst may be below the temperature at which it can readily ignite the mixture of carbonaceous fuel and combustion air fed to it, especially since this mixture may have a fuel-air ratio near or outside the limits of flammability under the conditions prevailing at the entrance to the catalyst. Until the temperature of the catalyst has been raised past the ignition temperature and preferably to a temperature approaching or at the final or steady-state operating temperature of the catalyst, it is highly desirable to provide means for assuring complete combustion of most or substantially all of the catalyst effluent which may be incompletely combusted. The proportion of incompletely combusted carbonaceous fuel found in the catalyst effluent during the warm-up period may be augmented, when using a liquid fuel such as No. 2 fuel oil, by failure to obtain an intimate admixture of the fuel and the combustion air prior to entry into the catalyst, caused by lack of sufficient preheating of the fuel-air feed during the warm-up period. In some combustion systems the preheating of the feed is obtained by heat transfer from the effluent of the combustion apparatus, or by compression of the combustion air as in a turbocharger driven by the combustion effluent. Since these preheating arrangements may be relatively ineffective during start-up of the combustion apparatus, an auxiliary source of heat may be required to effect start-up and attainment of a catalyst temperature within the operating range in a reasonably short length of time.
In U.S. patent application Ser. No. 645,017, filed Dec. 29, 1975, in the name of Asmund A. Boyum and assigned to the same assignee as that of the present invention, a method of combusting carbonaceous fuel is disclosed and claimed which involves combusting a first mixture of carbonaceous fuel and air in the presence of a catalyst to produce a catalyst effluent, providing another carbonaceous fuel-containing component differing from the first mixture, and mixing the catalyst effluent and such fuel-containing component to form a second mixture which is capable of homogeneous combustion to produce a hot effluent. Such a method may be useful in effecting efficient combustion of carbonaceous fuel of a type difficult to burn or undesirable for burning in the presence of a catalyst. In general, the combustion method of the aforementioned application is useful in continuous operation and requires the availability of a catalyst effluent from a catalyst combustion stage in which the catalyst has reached a temperature within its ordinary operating range. Such a method also may require preliminary start-up of the catalyst stage, and the method and apparatus of the present invention may be utilized for facilitating and improving operation during warm-up of the catalyst used in the method of the Boyum application.